It all depends on what you intend it to do. If you run heavily threaded software the opteron can´t compete with the Pentium D. This is still true if both processors are overclocked. Since most games arene´t running multiple threads, the opteron would be a lot better at games than the Pentium D though.
As for overclocking, i believe they are both easy to overclock, but the Pentium D will need more power and later on it will require more drastic cooling measures.